The invention relates generally to the semiconductor art and, more particularly, to an improved method and apparatus for forming thin film photovoltaic cells employing multinary materials, such as I-III-VI.sub.2 Cu-ternary chalcopyrite compounds. The compounds of particular interest in this area include, without limitation, CuAlS.sub.2, CuGaSe.sub.2, CuInS.sub.2, CuInSe.sub.2 and CuInTe.sub.2. Of these, CuInSe.sub.2 is the most promising.
In the relatively recent past, considerable interest has been directed to the use of CuInSe.sub.2 in photovoltaic devices. The following publications and patent relate to two recent efforts in this area:
1. Chen and Mickelsen, "Thin Film CdS/CuInSe.sub.2 Heterojunction Solar Cell", SPIE, Volume 248, Pages 62-69 (1980);
2. Mickelsen et al. U.S. Pat. No. 4,335,266;
3. Piekoszewski, et al., "Rf-Sputtered CuInSe.sub.2 Thin Films", Conference Record, 14th IEEE Photovoltaic Specialists Conference, pages 980-985 (1980);
Although CuInSe.sub.2 is theoretically promising for use in photovoltaic cells, most of the prior thin-film devices incorporating it have proven relatively inefficient. As pointed out in the article by Chen and Mickelsen, the inefficiency is believed to stem in large part from the existence of imperfections, such as voids, vacancies and nodules of copper, at the junction of p-type CuInSe.sub.2 with n-type CdS in the devices. Specifically, it has been found that when a conventional low resistivity (less than 50K ohms per square) p-type CuInSe.sub.2 film is exposed to CdS, a large number of nodules of pure copper are produced at the junction between the layers. Solar cells containing nodules of this type have exhibited low solar to electrical energy conversion efficiency.
In order to avoid the formation of nodules, Chen and Mickelsen deposited CuInSe.sub.2 films in two steps in the fabrication of CuInSe.sub.2 /CdS cells. Cu, In and Se were deposited by co-evaporation to form each of the two layers, with the deposition rate of Cu adjusted between the two layers in a single step. A low resistivity layer (less than 50K ohms per square) was deposited to a depth of approximately one micron, with a second layer of much higher resistivity deposited thereover to complete the selenide film. The change in resistivity was accomplished by adjusting the Cu evaporation rate during deposition. A substrate temperature of 450 degrees Celsius was attained during this process, presumably to aid in formation and diffusion of the composition-graded selenide film. A film of CdS was then deposited over the high resistivity portion of the selenide film to form a thin film solar cell reportedly having a relatively high efficiency.
In the SPIE article, Chen and Mickelsen disclose that the exact nature of the composition gradient produced by their method is determined by the relative Cu and In deposition rates, the relative layer thicknesses and the substrate temperature. The last of these variables is stated to be important since diffusion processes are active at high substrate temperatures.
Although the efforts of Chen and Mickelsen have yielded good experimental results with laboratory devices, the process is difficult to control and does not lend itself to efficient large scale production. In particular, the process involves thermal evaporation, which is very difficult to control in multi-component depositions. Evaporation is basically a boiling process which proceeds rapidly at rates proportional to the vapor pressure of each component. The system also possesses substantial inertia, causing it to respond slowly to changed input. However, a small change in input power eventually produces a significant change in evaporation rate. According to Chen and Mickelsen, deposition of CuInSe.sub.2 by thermal evaporation also involves substrate temperatures on the order of 450 degrees Celsius, presumably to foster crystal growth. Temperatures of this magnitude severely constrain a production environment and increase the cost of the process. Finally, streams of Cu, In and Se produced by thermal evaporation are difficult to controllably mix during deposition to control composition of the deposited film. The Cu, In and Se approximate point sources and have considerably different diffusion properties. The sources are therefore placed in complex spatial relationships with each other in an attempt to achieve adequate mixing. One possible source configuration is disclosed in the SPIE article. Elaborate devices are required for continuously sensing and controlling evaporation.
As described in the article by Piekoszewski et al., the compound CuInSe.sub.2 has been deposited onto a substrate by radio-frequency or "rf" sputtering to form a thin homogeneous film of a photovoltaic device. Rf-sputtering is a method by which ions from an rf-excited plasma are caused to impact a source or target material, vibrating atoms of the source material loose for deposition on a substrate. The atoms possess high kinetic energy and are intercepted by the substrate to form a deposited film.
The device described in Piekoszewski et al. was produced by rf-sputtering CuInSe.sub.2 from a specially prepared single source or target, and subsequently depositing a CdS film thereon. The single source of CuInSe.sub.2 made it impossible to vary the Cu/In ratio during deposition, giving the deposited film a uniform CuInSe.sub.2 composition. The device made with the film exhibited a low energy conversion efficiency, presumably due to Cu nodule formation and other defects. Rf-sputtering is poorly suited to use in large scale commercial systems, due to the level of rf power which must be applied to the source material to induce sputtering. Power on the order of 100 KW is usually required to generate a plasma and produce large scale rf sputtering. While this level of power can be accommodated in a laboratory situation such as that described by Piekoszewski et al., it is difficult to set up and control in a production sputtering system. For example, the transfer of high levels of rf power to a sputtering cathode is an inherently inefficient process. The impedance match between the power generator and the load is very critical, and there is always some power reflected back to the generator. Also, there are significant losses due to capacitive coupling between an rf cathode and ground. These problems are serious at high power levels, and are responsible for the fact that rf-sputtering has not been used in large scale production.
A process known as planar magnetron sputtering, also called magnetically enhanced sputtering, has been used to produce sputtering films of uniform composition in the manufacture of certain electronic devices and the application of reflective coatings to glass. The current level of technology in the magnetron sputtering field is evident from the following-listed article and United States patents: Chapin, "The Planar Magnetron", Research/Development, pages 37-40, January 1974; U.S. Pat. Nos. 3,878,085, 3,956,093, 4,060,470, 4,100,055, 4,116,806 and 4,162,954.
Magnetron sputtering involves the use of a magnetic field to trap electrons above a source, forming an intense plasma from which positive ions are drawn into impact with the source. The intensity of the plasma, and thus the rate at which ions collide with the source to produce sputtering, can be controlled accurately by controlling the electric power applied to the source.
Most of the magnetron sputtering devices and methods of which applicant is aware relate to sputtering a film of uniform composition. However, any CuInSe.sub.2 film deposited in the first instance as a film of uniform composition would necessarily exhibit the above-referenced Cu Nodule formation when used in conjunction with a CdS film. It is also believed that magnetron sputtering techniques have not been used to deposit CuInSe.sub.2 or any other ternary chalcopyrite compound employing materials selected from the chemical groups I, III and VI, respectively.
Anderson et al., "Magnetron Reactive Sputtering Deposition of Cu.sub.2 S/CdS Solar Cells", Proceedings, 2nd European Community Photovoltaic Solar Energy Conference, Pages 890-897 (1979), discloses a system in which a number of magnetron sputtering cathodes are used to sequentially deposit films of Cu.sub.2 S, CdS and Nb. The cathodes are operated separately for sequential deposition of Cu.sub.2 S and CdS in a reactive atmosphere containing S. The only instance in which two of the cathodes are operated simultaneously is the deposition of In-doped CdS films by co-sputtering Cd and In targets in a reactive H.sub.2 S atmosphere. Indium was used only as a dopant in the production of films of uniform concentration, and was present in extremely small quantities. Under these circumstances, it is believed that the parameters of mixing are much less critical than in the case of depositing the major constituent elements of a highly composition-dependent multinary material, such as CuInSe.sub.2.
U.S. Pat. No. 4,322,276 to Meckel et al. discloses a method of forming a thin film refractive coating wherein the composition of a metal and its oxide within the film varies as a function of depth. Deposition is accomplished by passing a continuous flexible substrate over a series of magnetron sputtering cathodes containing different compositions of the metal and the oxide. The cathodes are separated for sequential deposition onto the passing substrate, except for a small area of overlap.
U.S. Pat. No. 4,278,528 to Kuehnle et al. discloses a system in which a number of materials are deposited as discrete parallel strips from a plurality of rf sputtering cathodes.
Japanese Pat. No. 55-12732 to Funaki discloses a variety of sputtering cathode structures containing more than one material. However, it appears that each of the structures is a single electrically conductive unit, preventing individual control of the rates at which the materials are sputtered. Therefore, they would not be suitable for depositing composition-graded films of multinary materials.
Finally, mention is made in Leong et al., "Advances in the SERI/DOE program on CdS/Cu.sub.2 S and CdS/Cu-ternary Photovoltaic Cells", 15th IEEE Photovoltaic Specialists Conference, pages 1016-1020, August 1981, of reactive magnetron sputtering. The article describes cells fabricated by thermal evaporation processes, and lists reactive magnetron sputtering as one of the other methods currently being pursued "within the CdS program". However, the discussion following the list, and the data entries of Table 2, describe sputtering only in connection with a CdS/Cu.sub.2 S cell in which each layer is generally homogeneous. The layers are formed by reactive sputtering of the metallic element (Cd or Cu), in an atmosphere containing H.sub.2 S. The article therefore appears to be irrelevant to codeposition of a number of constituent elements of a multinary semiconductor, such as CuInSe.sub.2, in composition-graded form. In any event, it is believed that the Leong et al. publication is not prior art to the present invention because it was published after conception of the present invention and applicants exercised diligence in reducing the invention to practice.
Therefore, it is desirable to provide an improved method and apparatus for accurately and controllably depositing a high quality composition-graded film of a multinary semiconductor material, such as CuInSe.sub.2.